Vinylidene fluoride (VDF) polymers are known in the art to be suitable as binders for the manufacture of electrodes for use in non-aqueous-type electrochemical devices such as batteries, preferably secondary batteries, and electric double layer capacitors.
Generally, techniques for manufacturing electrodes involve the use of organic solvents such as N-methyl-2-pyrrolidone (NMP) for dissolving VDF polymer binders and homogenizing them with a powdery electrode material and all other suitable components to produce a paste to be applied to a metal collector.
The role of the organic solvent is typically to dissolve the VDF polymer in order to bind the powdery electrode material particles to each together and to the metal collector upon evaporation of the organic solvent.
Nevertheless, more recently, approaches are pursued wherein use of organic solvents is commonly avoided so as to ensure more environmentally friendly techniques.
For instance, US 2002/0168569 (ATOFINA) 14, Nov. 2002 discloses a process for manufacturing electrodes and separators for lithium ion batteries, said process comprising processing a microcomposite powder obtained from a fluoropolymer such as notably an acrylic modified fluoropolymer in the form of solid particles having a size between 0.1 μm and 0.5 μm and fillers. The microcomposite powder may be processed without solvents or with solvents by redispersing this powder either in water or in a latent solvent such as acetone or N-methyl-2-pyrrolidone.
Also, US 2006/0099505 11, May 2006 discloses a process for manufacturing an anode for a battery comprising using an anode mixture slurry prepared by dispersing a particulate anode active material and a particulate binder containing at least one of the group consisting of homopolymers and copolymers of vinylidene fluoride (VDF) in a dispersion medium having a swelling degree of 10% or less to the binder. Representative examples of particulate binders typically have an average particle diameter of preferably 30 μm or less and notably include copolymers of VDF with an ethylene unsaturated monomer such as acrylic ester, methacrylic acid ester, vinyl acetate, acrylonitrile, acrylic acid, methacrylic acid, mallein anhydride, butadiene, styrene, N-vinyl pyrrolidone, N-vinyl pyridine, gycidyl methacrylate, hydroxyethyl methacrylate, methyl vinyl ether. The dispersion medium is preferably water, ethanol or methyl isobutyl ketone.
Nevertheless, the processes described in these documents require isolating and drying a vinylidene fluoride (VDF) polymer powder prior to its redispersion in water.
Also, waterborne binder systems have been developed such as those described in US 2010/0304270 (ARKEMA INC.) 2, Dec. 2010, wherein aqueous vinylidene fluoride (VDF) polymer binder latexes as synthesized are used for manufacturing electrodes for non-aqueous-type electrochemical devices.
However, the waterborne vinylidene fluoride (VDF) fluoride polymer binder compositions of the prior art still do not enable obtaining electrodes ensuring good cohesion within the particulate active material and good adhesion between these particles and the metal collector.
To effectively employ waterborne polymer binder compositions in electrode-forming processes, it is important to develop binder systems notably having sufficient stability prior and after admixing with the powdery electrode material, suitable filmability and processing characteristics for being advantageously applied onto the metal collector via standard techniques and for yielding a uniform distribution of the powdery electrode material within the electrode. Most importantly, these binder systems should provide proper cohesion within the powdery electrode material particles and proper adhesion of the powdery electrode material particles to the metal collector after drying.
It is understood that the polymer binder should properly bind the electrode material particles together and to the metal collector so that these particles chemically withstand large volume expansion and contraction during charging and discharging cycles, especially in case of negative electrodes.
A poor cohesion within the powdery electrode material particles and an insufficient adhesion of these particles to the metal collector are typically responsible for high electrical resistance, low capacity and low ion mobility within the electrode.
Further, to be advantageously used in the manufacture of electrodes, in particular for batteries, it is required that the polymer binder is chemically resistant to the electrolyte present in the battery.
There is thus still a need in the art for aqueous vinylidene (VDF) polymer polymer binder compositions which advantageously enable easily manufacturing electrodes for use in non-aqueous-type electrochemical devices, said electrodes having enhanced adhesion and cohesion within the powdery electrode material particles to be endowed with outstanding capacity values.